The term steampunk means a lot of things. The first aspects of the genre that generally come to mind involve mostly Victorian era fashion mixed with unnecessary smithing goggles as a fashion accessory coupled with clockwork backdrops and cleverly bronze-looking re-imaginings of modern technology. Steampunk as a genre in fiction doesn’t generally have a lot of steam-powered computers, but the name does imply steam and there are definitely things that are steam-powered, despite the clockwork aesthetic being more prevalent than the actual steam usage.
One of the long-standing symbols of steampunk has been dirigibles (or zeppelins), which show up in everything from Homunculus to Boneshaker to Leviathan and everything in between. Whether or not these flying machines are actually supposed to be steam-powered or they are just a cool ship that feels very Victorian is unclear, but it is worth examining the idea of a zeppelin completely powered by steam and what would actually have to go into making such a thing a reality.
The basic premise of using steam as an energy source is that the steam is hot and capable of creating pressure and thus is creating energy much in the way a combustion engine creates pressure that move pistons. The problem is that to generate the steam to begin with there needs to be some kind of external power source to create the reaction that actually generates steam. If you want to steam hot dogs, you don’t just throw hot dogs on the steamer– you have to either turn it on with electricity or put it on a stove-top and heat the water to generate the steam. This makes problem one basically that without an external energy source, the steam couldn’t work, so therefore it would only be steam-powered in the way that a steam engine is. Nothing is just steam, there is always something else.
Steam still accounts for an astounding majority of our power generation. It is well known how important the use of steam as a power source was to modernization and the beginning of the Industrial Revolution, with things like steam locomotives and otherwise giving an energy source that was easy to get as long as you had coal to burn to start the reaction. It seems that for a zeppelin to work it would require a calculation of the amount of weight the machine has to lift (including passengers, contents, and the weight of the ship itself) and make an engine large enough to generate that amount of pressure (note: this is a lot) and somehow both fit it on the ship and make sure to account for the weight of the engine itself in calculating how much energy it needs to generate. It is possible, but so entirely impractical that it would be just about impossible without using a whole lot of modern technology and thus defeating the purpose.
Is any of this important? No. There really is no such thing as a hard science fiction steampunk, unless it were little more than people in Victorian England making clocks and doing nothing at all extravagant, which leaves you with Victorian literature, not steampunk at all. Space travel in the future can be written about with some degree of realism, but you will have a hard time finding steampunk that is anything more then fantasy, and that is just fine.